We want to abstract two primary principles and processes from evolution.
Randomization means that a given trait or mechanism is not repeated exactly through all individuals in the species. This randomization must include variations that are inheritable, not just 'errors'. Producing such randomized dispersion must therefore be one of the basic processes for evolution.
Selection means that some of the inheritable random variations are more successful in allowing the individual with those variations to survice and reproduce. Over time, over successive generations, the selection process will result in increasing the percentage of the successful variations in subsequent populations.
The combination of randomization and selection allows the whole population of the species to change, even though single individuals are not changed. This dual and balancing process of randomization and selection gives the appearance of the species learning to adapt to its environment.